versions: Query and Install Specific Versions of Packages on CRAN

Installs specified versions of R packages hosted on CRAN and
provides functions to list available versions and the versions of currently
installed packages. These tools can be used to help make R projects and
packages more reproducible. 'versions' fits in the narrow gap between
the 'devtools' install_version() function and the 'checkpoint' package.
devtools::install_version() installs a stated package version from source files
stored on the CRAN archives. However CRAN does not store binary versions of
packages so Windows users need to have RTools installed and Windows and OSX
users get longer installation times. 'checkpoint' uses the Revolution Analytics
MRAN server to install packages (from source or binary) as they were available
on a given date. It also provides a helpful interface to detect the packages
in use in a directory and install all of those packages for a given date.
'checkpoint' doesn't provide install.packages-like functionality however, and
that's what 'versions' aims to do, by querying MRAN. As MRAN only goes back to
2014-09-17, 'versions' can't install packages archived before this date.